PARKLAND, Fla. — Delaney Tarr, a high school senior, cannot remember a time when she did not know about school shootings.

So when a fire alarm went off inside Marjory Stoneman Douglas High School and teachers began screaming “Code red!” as confused students ran in and out of classrooms, Ms. Tarr, 17, knew what to do. Run to the safest place in the classroom — in this case, a closet packed with 19 students and their teacher.

“I’ve been told these protocols for years,” she said. “My sister is in middle school — she’s 12 — and in elementary school, she had to do code red drills.”

This is life for the children of the mass shooting generation. They were born into a world reshaped by the 1999 attack at Columbine High School in Colorado, and grew up practicing active shooter drills and huddling through lockdowns. They talked about threats and safety steps with their parents and teachers. With friends, they wondered darkly whether it could happen at their own school, and who might do it.

Now, this generation is almost grown up. And when a gunman killed 17 people this week at Stoneman Douglas High in Parkland, Fla., the first response of many of their classmates was not to grieve in silence, but to speak out. Their urgent voices — in television interviews, on social media, even from inside a locked school office as they hid from the gunman — are now rising in the national debate over gun violence in the aftermath of yet another school shooting.

While many politicians after the shooting were focused on mental health and safety, some vocal students at Stoneman Douglas High showed no reluctance in drawing attention to gun control.

They called out politicians over Twitter, with one student telling Senator Marco Rubio, a Florida Republican, “YOU DON’T UNDERSTAND.” Shortly after the shooting, Cameron Kasky, a junior at the school, and a few friends started a “Never Again” campaign on Facebook that shared stories and perspectives from other students who survived the rampage.

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Just hours after 17 people were killed in a mass shooting at their high school in Parkland, Fla., students turned to social media to advocate for more gun control.

On a day when the funerals of the shooting victims began here, more than a dozen schools from Massachusetts to Iowa to Michigan were shut down in response to copycat threats and social media interpreted in the worst light. A college near Seattle was on lockdown for several hours on Friday after an unfounded report of gunfire and in at least one case an entire district closed down. Several students have been arrested, accused of phoning in threats to their schools.

At other high schools across the country, students rallied in solidarity with Stoneman Douglas High and staged walkouts to protest what they called Washington’s inaction in protecting students and teachers. A gun control advocacy group, Moms Demand Action, said it had been so overwhelmed with requests from students that it was setting up a parallel, student focused advocacy group.

His argument reflects the words of other students who want action: The issue is not an abstraction to them. These are their murdered friends, their bloodstained schools, their upended lives.

Students said they did not want to cede the discussion over their lives to politicians and adult activists.

“We need to take it into our hands,” Mr. Kasky said.

David Hogg, a 17-year-old student journalist who interviewed his classmates during the rampage in Parkland, said he had thought about the possibility of a school shooting long before shots from an AR-15 started to blast through the hallways. As he huddled with fellow students, he stayed calm and decided to try to create a record of their thoughts and views that would live on, even if the worst happened to them.

“I recorded those videos because I didn’t know if I was going to survive,” he said in an interview here. “But I knew that if those videos survived, they would echo on and tell the story. And that story would be one that would change things, I hoped. And that would be my legacy.”

It is a stark change from the moments that followed the Columbine shooting in April 1999, said Austin Eubanks, who survived the shooting. Mr. Eubanks and a friend hid under a table when the two teenage gunmen walked into the library and started shooting. Mr. Eubanks was wounded. His friend, Corey DePooter, was killed.

“There was nobody who took an activism stance,” Mr. Eubanks said of Columbine’s immediate aftermath. He said he began abusing opiates shortly after as a coping mechanism. “I just wanted to be left alone. I was so destabilized and traumatized.”

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Nathanael Clark, a Stoneman Douglas High student, and his father, John, talked to the news media after Wednesday’s shooting.CreditSaul Martinez for The New York Times

Mr. Eubanks now helps run an addiction treatment center in Colorado and has sons of his own, aged 8 and 12. The oldest has asked why Columbine happened and whether he needs to be afraid, and Mr. Eubanks said he has tried to make the boys feel safe while also discussing how children can drift toward violence.

No matter how rare school shootings are for the vast majority of students, they have grown up in a world so attuned to these threats that high schoolers are now more conversant in the language of lockdowns and code red drills than their parents.

Spencer Collier, the police chief in Selma, Ala., was chatting recently with a group of high school students when they brought up mass shootings and pressed him about current trends and what law enforcement agencies were doing to address them. In Connecticut, Nathaniel Laske, a high school junior, said he had asked school administrators about the apparent absence of lockdown drills or a mass shooting plan in the event something happened during school theater productions.

“A lot of people aren’t willing to talk about it,” Mr. Laske said. “When you’re part of a school community it makes you much more inclined to want to prevent things.”

Soon after Amy Campbell-Oates, 16, heard about the Parkland shooting, she knew she wanted to try, in some small way, to influence the national discussion on gun violence. She and two friends organized a protest, made posters, and on Friday, they rallied with dozens of fellow students from South Broward High School.

They carried signs that read “It Could’ve Been Us,” and “Your Silence is Killing Us,” and “We Stand with Stoneman Douglas.” They chanted, their collective voices rising as cars honked in support.

“We agreed that our politicians have to do more than say thoughts and prayers,” Ms. Campbell-Oates said. “We want voters to know that midterms are coming up. Some of us can’t vote yet but we want to get to the people that can to vote in common sense laws, ban assault rifles and require mental health checks before gun purchases.”

Tyra Hemans, a senior at Stoneman Douglas High, made a poster, too, emblazoned with the word “ENOUGH.” On Friday, Ms. Hemans attended the funeral for Meadow Pollack, one of the 17 people killed, and then she spoke about her desire to see President Trump when he visits the area.

“I want our politicians to stop thinking about money and start thinking about all these lives we have lost,” she said. “I want to talk with him about changing these laws. Seventeen people are dead, killed in minutes.”

Correction:Feb. 17, 2018

An earlier version of this article misstated the number of students killed in the shooting at Marjory Stoneman Douglas High School. The gunman killed 17 people: 14 students and three faculty members. He did not kill 17 students.

A version of this article appears in print on , on Page A1 of the New York edition with the headline: ‘Code Red!’ Mass Shooting Generation Raises Voices for Change. Order Reprints | Today’s Paper | Subscribe